i'm really into the halo world and this one made me h3pp1 f31c.
now i think that's pretty good as a simple toon, but i see some great potential in your skills and since you're such a great halo fan you could be the perfect machine.
you see, you mix two techniques (toony black outline and smooth color shading), but to me you should concentrate on only one for each piece, because the visual impact of your smooth coloring isn't as strong as the outline and can't fit with it. viceversa the black outline can't fit in a soft colored piece, as it ruins all the shading effects. hope it helped.